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Re: Question about _dl_debug_state and new glibc
- From: Mark Kettenis <mark dot kettenis at xs4all dot nl>
- To: drow at false dot org
- Cc: kevinb at redhat dot com, randolph at tausq dot org, gdb at sources dot redhat dot com, libc-alpha at sources dot redhat dot com
- Date: Mon, 11 Oct 2004 21:13:41 +0200 (CEST)
- Subject: Re: Question about _dl_debug_state and new glibc
- References: <20041011172207.GA14151@tausq.org> <20041011104647.6a03d9ce@saguaro> <20041011175524.GA29450@nevyn.them.org>
Date: Mon, 11 Oct 2004 13:55:25 -0400
From: Daniel Jacobowitz <drow@false.org>
On Mon, Oct 11, 2004 at 10:46:47AM -0700, Kevin Buettner wrote:
> On Mon, 11 Oct 2004 10:22:07 -0700
> Randolph Chung <randolph@tausq.org> wrote:
>
> > I've been away from gdb for a bit, so hopefully this is not something
> > simple I missed while I've been away :)
> >
> > It appears that newer versions of glibc now marks the _dl_debug_state
> > function as hidden, making it not visible to gdb. This breaks the
> > solib tracking code in solib-svr4.c.....
> >
> > Looking at the docs, it looks like the "proper" way for gdb to do this
> > is to look up the r_debug symbol and use the r_brk member to locate
> > _dl_debug_state's address. is there any particular reason why we don't
> > do this in gdb?
Only for static executables usually; for dynamic executables it's
supposed to be DT_DEBUG if that's available. Same difference.
Hey wake up Daniel. Static executables don't have shared libraries.
> I can't think of any reason. I'm guessing that it was more expedient
> for the original authors of the code to do it the way it was done.
> In any case, I know of no reason not to change it so that it's done
> "properly".
We set the breakpoint on _dl_debug_state before starting the inferior.
_dl_debug_initialize initializes _r_debug somewhat later... I'm not
sure how it is supposed to be used. Do either of you know?
The problem is that even DT_DEBUG isn't initialized before
initialization of the dynamic linker. Theoretically, we could put a
watchpoint on DT_DEBUG, and lookup r_brk if it triggers. However, I
don't think that's very practical on platforms without hardware
watchpoint support.
I suppose this is just something that has never been completely
implemented. Somehow I think the dynamic linker was supposed to trap
just after initializing r_debug and setting DT_DEBUG if it was being
traced, perhaps if it noticed that DT_DEBUG was set to some special
value by the debugger. Perhaps we'll know when "Open Solaris" is
released.
I see it hasn't been exported since March. Blech.
So we should ask the glibc developers to unhide _dl_debug_state.
Mark